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Description: While the importance of canopy structure in open woodlands and savannas on regulating the flow of energy and matter is well known, few studies have investigated how variation in overstory abundance influences canopy light transmission and the extent that estimates vary in their ability to characterize the light environment in these ecosystems. Canopy light transmittance (% photosynthetic photon flux density, or %PPFD) was measured with gallium arsenide phosphide (GaAsP) photodiodes and was monitored throughout the growing season in an open-canopy longleaf pine (Pinus palustris Mill.) woodland across an overstory abundance gradient. Several estimates of canopy light transmittance were also measured, including 10 minute averages of %PPFD under clear and overcast sky conditions during summer, fall, and winter, as well as light estimates derived from hemispherical photographs (gap fraction, GF: gap light index, GLI; and weighted canopy openness, WCO). Linear and curvilinear regression were used to analyze the relationship between (1) light measurements and canopy structure and (2) light estimates and growing-season % PPFD measured by light diodes. Of all light measurements in this study, ten-minute average % PPFD measurements during clear days were the most variable: only a small proportion in the 10-minute average of light reaching the understory was correlated to either overstory structure or growing-season canopy transmittance. While measures on overcast days improved the correlations, they tended to overestimate transmittance. Gap fraction (GF) and gap light index (GLI) were strongly correlated with % PPFD and overstory structure, and the relationship between GLI and % PPFD was improved by using a direct light setting of 60%. In addition to strong correlation, hemispherical photographic estimates of light fell along a 1:1 line with % growing-season PPFD; thus, estimates were relatively unbiased. Measures of light in this ecosystem need to incorporate the large spatial and temporal variation in light transmittance that is characteristic of this ecosystem if investigators are to understand the relationship between open-canopy woodlands and their light environments.